1. Field of the Invention
This invention relates to method and apparatus for introducing geometrical perturbations along an optical fiber waveguide and, more particularly, to method and apparatus for introducing mode-coupling and geometric perturbations along optical fiber waveguides using a modulated heat source.
2. Description of the Prior Art
Modal dispersion in a multimode optical waveguide can be reduced by deliberately enhancing the mode coupling process in the waveguide. Such mode coupling can be accomplished, for instance, by introducing controlled imperfections or perturbances into the waveguide which tend to induce coupling among the various propagating modes thereby resulting in a transmitted pulse arriving at the other end of a guide as a single, slightly broadened, pulse.
It is known to form perturbation patterns which change the cross-sectional dimensions and/or the direction of the axis of an optical fiber waveguide as it is being drawn to enhance coupling between either pairs of propagating modes or all of the propagating modes by positioning one or more piezoelectric members in contact with the base of a preform opposite the end being heated and drawn. The piezoelectric members are selectively energized from a single source to produce the desired perturbations. In this regard, see U.S. Pats. No. 3,666,348 issued to E. A. J. Marcatili on May 10, 1972, and U.S. Pat. No. 3,687,514 issued to S. E. Miller et al on Aug. 29, l972.
U.S. Pat. No. 3,499,194 issued to H. J. Schmick on Mar. 10, 1970 relates to a method of glass texturizing wherein a glass filament raised to a temperature within the working temperature range of the glass is permanently waved by passing the filament through an orifice oscillating laterally to the direction of the filament travel. The waves thus formed in the filament are accumulated, compacted and cooled to a temperature below the working temperature.
U.S. Pat. No. 3,912,478 issured to H. M. Presby on Oct. 14, 1975, relates to methods for introducing geometric variations in optical fibers. There, one or more inert gas streams are directed against a fiber as it is pulled from a heated preform. The gas, whose force against the fiber is modulated, serves to displace the fiber in a direction transverse to the pulling direction of the fiber axis and the fiber diameter. Various means are also disclosed, such as valving systems, for properly modulating the force intensity of the gas against the heated fiber.